This video shows you how to add images to your WordPress site, how to find the correct sizes and also advice on resizing and optimising images.
You may notice when creating your layouts, expecially if you have a row of images, that they are all different sizes. In WordPress you need to upload images in a certain size before putting them on the page. So for example, if you want to have a row of three images the same size then they need to be identical heights and widths BEFORE you upload them.